In recent years, the measurement of tiny vibration amplitude is becoming possible with the user of a laser Doppler principle in a non-contact manner. In the laser Doppler principle, Doppler shift occurs in scattering light if a vibrating object is irradiated with laser light. Information such as vibration amplitude generated in the object can be obtained by observing light beat (interference) between the scattering light and reference light.
A detection method is proposed which applies vibration to a detection object actively and grasps a state of the object by observing the vibration of the object with the use of the laser Doppler principle (see Japanese Unexamined Patent Application Publication No. 2012-237561).
On the other hand, a technology is proposed which vibrates an object with the sound wave having a sharp directivity by using ultrasonic wave like a parametric speaker as a module that vibrates an object in a non-contact manner (see Japanese Unexamined Patent Application Publication No. 2014-106102).
The technology disclosed in Japanese Unexamined Patent Application Publication No. 2014-106102 comprises a wave transmission section for outputting sound wave towards an object and an optical measurement section for optically measuring vibration. The wave transmission section and the optical measurement section are arranged at optional positions for use (physical positions thereof are different from each other). Thus, it is necessary to re-adjust an angle of the wave transmission section according to a distance and an angle between the optical measurement section and a measurement object. It takes much time and effort to obtain lots of data and to realize normalization.